Components of combustion turbines are routinely subjected to harsh environments that include rigorous mechanical loading conditions at high temperatures, high temperature oxidization, and exposure to corrosive media. The structural stability of such components is often provided by nickel or cobalt base superalloys, for example, due to their exemplary high temperature mechanical properties such as creep resistance and fatigue resistance.
Creep is the term used to describe the tendency of a solid material to slowly move or deform permanently to relieve stresses. It occurs as a result of long-term exposure to levels of stress that are below the yield strength or ultimate strength of the material. Creep is more severe in materials that are subjected to heat for long periods and near their melting point, such as alloys out of which combustion turbine components are formed. If a turbine blade, for example, were to deform so that it contacted the turbine cylinder, a catastrophic failure may result. Therefore, a high creep resistance is an advantageous property for a combustion turbine component to possess.
Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. Given the numerous fatigue cycles a combustion turbine component may endure, a high fatigue resistance is likewise an advantageous property for a combustion turbine component to possess.
One way to strengthen a material, enhancing both its creep resistance and its fatigue resistance, is known as dispersion strengthening. Dispersion strengthening typically occurs by introducing a fine dispersion of particles into a material, for example, a metallic component. Dispersion strengthening can occur by adding material constituents that form particles when the constituents are added over their solubility limits.
Alternatively, dispersion strengthening may be performed by adding stable particles to a material, in which these particles are not naturally occurring in the material. These particles strengthen the material and may remain unaltered during metallurgical processing. Typically, the closer the spacing of the particles, the stronger the material. The fine dispersion of close particles restricts dislocation movement, which is the mechanism by which creep rupture may occur.
Previous dispersion strengthening methods include the introduction of thoria, alumina, or yttria particles into materials out of which combustion turbine components are formed. Thoria, alumina, and yttria are oxides of rare-earth elements that possess a higher bond energy than oxides of metals such as iron, nickel, aluminum, or chromium that are typically used as the base metal of combustion turbine components.
For example, U.S. Pat. No. 5,049,355 to Gennari et al. discloses a process for producing a dispersion strengthened alloy of a base metal. A base metal powder and a powder comprising thoria, alumina, and/or yttria are pressed into a blank form. The pressed blank form is sintered so that the thoria, alumina, and/or yttria are homogenously dispersed throughout the base metal.
U.S. Pat. No. 5,868,876 to Biano et al. discloses a process for producing a creep resistant molybdenum alloy. A slurry of molybdenum oxide and an aqueous solution of lanthanum, cerium, and/or thoria is formed. The slurry is heated in a hydrogen atmosphere to produce a metallic powder. The powder is pressed then sintered. Finally, the sintered powder is thermomechanically processed to produce a molybdenum alloy having an oxide dispersion of lanthanum, cerium, and/or thoria.
U.S. Pat. No. 6,231,807 to Berglund discloses a method of producing a dispersion hardened FeCrAl alloy. A starting powder including iron, chromium, and titanium and/or yttrium is mixed with a chromium nitride powder. The powder mixture is placed into an evacuated container and heat treated. During heat treatment, titanium nitride is formed in a mix of chromium and iron. The nitrided chromium and iron product is then alloyed with aluminum by a conventional process to form a dispersion strengthened FeCrAl alloy.
The pursuit of increased combustion turbine efficiency has led to increased turbine section inlet temperatures, and thus metallic components made from different materials and having increased high temperature creep and fatigue resistance may be desirable.